1. Field of the Invention
The invention relates to a method of transmitting frames, i.e. packets, in a telecommunication network and an equipment for implementing the method.
2. Description of the Related Art
The invention is explained in an exemplary manner in connection with a mobile communication system, but it can also be implemented in other kinds of telecommunication systems. FIG. 1 shows the parts of the mobile communication system that are essential for understanding the invention. Mobile Stations MS communicate with Base Transceiver Stations BTS1 and BTS2 over an air interface Um. The base transceiver stations are controlled by Base Station Controllers BSC associated with Mobile Switching Centres MSC. A subsystem controlled by a base station controller BSC, including the Base Transceiver Stations BTS controlled by the system, is called a Base Station Subsystem BSS. The interface between the exchange MSC and the base station subsystem BSS is called an A interface. The part of the mobile system that is on the MSC side of the A interface is called a Network Subsystem NSS. The interface between the base station controller BSC and the base transceiver station BTS, in turn, is called an Abis interface. The mobile switching centre MSC connects incoming and outgoing calls. It has similar functions as an exchange of a Public Switched Telephone Network PSTN. In addition to these, it also performs functions that are typical of mobile communication only, such as subscriber location management, in co-operation with the subscriber registers of the network, which in FIG. 1 are represented by a home location register HLR and a visitor location register VLR. The location of a mobile station MS is maintained in the visitor location register VLR with an accuracy of a Location Area LA. The size of the location area is one or more cells. When a mobile station MS observes that the location area has changed, it transmits a location update message to the network, according to which the new location area LA1, LA2 of the mobile station MS is updated in the visitor location register VLR, in whose area the mobile station MS is.
A conventional radio connection used in digital mobile communication systems is a circuit switched connection, which means that resources allocated to a subscriber are reserved for the connection concerned for the entire duration of the call. A General Packet Radio Service GPRS is a new service designed for digital mobile communication systems, such as the GSM system. The packet radio service is described in ETSI specifications TC-TR-GSM 02.60 and 03.60. The packet radio service makes it possible to offer the user of a mobile station MS a packet-form radio connection effectively utilizing radio resources. On a packet switched connection, radio resources are reserved only when speech or data is to be sent. The speech or data is collected in packets of a certain length. When a packet like this has been transmitted over the air interface Um, and the transmitting party does not immediately have a new packet to send, the radio resource can be released to other subscribers.
The system of FIG. 1 comprises a separate Serving GPRS Support Node or SGSN, which controls the operation of the packet data service on the network side. The control comprises, for example, logging of the mobile station on and off the system, location updating of the mobile station, and routing of the data packets to the correct destination. In the present application, xe2x80x98dataxe2x80x99 is interpreted widely to mean any information transmitted in a digital mobile system, for example speech encoded in digital form, data transmission between computers, or telefax data. An SGSN node can be located in connection with a base transceiver station BTS, a base station controller BSC or a mobile switching centre MSC, or it may be separate from them. The interface between an SGSN node and a base station controller BSC is called a Gb-interface.
Information, such as control signalling and speech or other data, is transmitted in the packet network by GPRS frames. Each frame F comprises a header 1 and a data part 2. The frames can be divided into two groups, or types, depending on whether they transport payload traffic to/from the user or signalling apart from the payload, the signalling being used to transmit control messages between network elements. Payload traffic is transported in Information Frames I and signalling in Unacknowledged Information Frames UI. Both I frames and UI frames comprise a cell ID, or else a network element, for example a base station located on the route of the frame, can insert the cell ID from which the SGSN node can conclude from which cell the frame was transmitted. The I frames comprise an internal numbering used in retransmitting I frames and in flow control, but the UI frames do not have such a numbering.
Problems in location updating may occur in a packet radio network complying with the above ETSI specifications. A problem may occur e.g. in the following situation (reference is made to FIG. 2). In step 2-1 the mobile station MS transmits data in a normal I frame, where the cell ID comprises an identifier of cell C1. Let us assume that the BSS (or another intermediate network element) does not have time to immediately transmit all low-priority frames but stores such frames in a queue. In step 2-2 the mobile station MS moves from cell C1 to cell C2. In step 2-3 the mobile station MS informs about the location update by transmitting a UI frame concerning the matter. In a BSS queue, a UI frame bypasses an I frame as the priority of the UI frame is higher. In step 2-4 the BSS (having the I frame received in step 2-1 in its queue) consequently transmits to the SGSN node the UI frame, which the mobile station MS transmitted in step 2-3. On the basis of this information the SGSN node concludes that the mobile station MS has moved to cell C2. In step 2-5 the SGSN node transmits to cell C2 data D addressed to the mobile station MS. In step 2-6 the BSS transmits the I frame it received in step 2-1. Based on this information, the SGSN node (incorrectly) concludes that the mobile station MS has returned to cell C1. In step 2-7, the SGSN node again transmits data D addressed to the mobile station. The latest information received by the SGSN node concerning the location of the mobile station MS is based on the I frame received in step 2-6, indicating that the mobile station is located in cell C1. Based on this information, the SGSN node transmits data addressed to the mobile station to cell C1, where the mobile station MS is, however, no longer located. This causes unnecessary delays and use of the resources of the network and the air interface, as the mobile station MS will, however, not be found in cell C1. Instead, it must be searched for and the data must be retransmitted until the mobile station is finally found in cell C2.
An object of the invention is thus to develop a method and an equipment for implementing the method in order to solve the problems of the above location update. The object of the invention is achieved with a method and a system which are characterized by what is disclosed in the characterizing part of the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.
The idea of the invention is that a general numbering sequence covering both the I and UI frames is inserted into the frames. As a result, the receiver is able to conclude the correct order of frames from the frame number and then avoid any incorrect location updates of prior art packet networks.